Incompatibility with logstash-logback-encoder version 7.1+

Opening an issue to track blocker for upgrading `logstash-logback-encoder` from version 7.0.1, for more visibility to context and past investigations in closed/superseded PRs.

Blocked PRs for upgrade to 7.3: #1600, #1601
Incompatibility from 7.1 bump: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/1065#issuecomment-1095229985
Proposed workaround from 7.2 bump: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/1138

```
Error: Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.10.1:compile (default-compile) on project spring-cloud-gcp-logging: Compilation failure: Compilation failure:
Error: /home/runner/work/spring-cloud-gcp/spring-cloud-gcp/spring-cloud-gcp-logging/src/main/java/com/google/cloud/spring/logging/extensions/LogstashLoggingEventEnhancer.java:[45,64] error: getFieldValue() has protected access in ObjectAppendingMarker
Error: /home/runner/work/spring-cloud-gcp/spring-cloud-gcp/spring-cloud-gcp-logging/src/main/java/com/google/cloud/spring/logging/extensions/LogstashLoggingEventEnhancer.java:[51,78] error: getFieldValue() has protected access in ObjectAppendingMarker
```

Start with spring-cloud-gcp-logging/src/main/java/com/google/cloud/spring/logging/extensions/LogstashLoggingEventEnhancer.java and reproduce the Maven compilation failure for logstash-logback-encoder 7.1+. Read the context in blocked PRs #1600 and #1601 and the linked investigations, including the proposed workaround in PR #1138. Done means upgrading to the newer encoder version without the protected-access compilation errors.

Written by the indexing model from the issue text.